Nowadays, the distributed voltage regulation (or distributed power supply) system, which has such advantages as high efficiency, high reliability, and a highly flexible input voltage range, is widely used as the power supply system of computers, computer peripherals, and many electronic instruments. As shown in FIG. 1, the circuit structure of a DPS system includes a front-end power factor correction (PFC) circuit 10 and a back-end DC/DC converter 11. The front-end PFC circuit 10 is a step-up circuit configured mainly for eliminating the harmonics of an AC input voltage of 85˜265V so as to obtain a stable DC voltage of 380˜400V. The back-end DC/DC converter 11, on the other hand, is configured mainly for isolating the DC voltage into DC voltages of different magnitudes for use by the internal components 12, 13, and 14 of an equipment or instrument.
Generally speaking, referring again to FIG. 1, in designing the front-end PFC circuit 10, a sufficient power supply hold-up time must be ensured so that a switched-mode power supply will not be shut down when an input AC power supply is turned off (or on) instantaneously. Furthermore, in designing the back-end DC/DC converter 11, it is necessary to ensure activation under high input voltage, and in order to stay efficient under high input voltage, power loss must also be minimized. These requirements, however, are not readily met by conventional DC/DC converters. Take the traditional pulse-width modulation (PWM) converter for example. A high input voltage not only lowers conversion efficiency but also makes a wide output voltage range unattainable. Moreover, as the power devices (e.g., metal-oxide-semiconductor field-effect transistors, or MOSFETs) of a conventional PWM converter are used as switches, and energy is delivered by means of switching, switching loss and electromagnetic interference (EMI) tend to aggravate as the switching frequencies in a unit time continuously increase.
To solve the aforesaid problems, resonant converters based on the soft-switching technology have emerged. The main principle of a resonant converter is to add such elements as resonant inductors and resonant capacitors to the primary side of a transformer via series-connection, parallel-connection, or series-parallel connection. Hence, by virtue of a resonant controller chip and the principles of resonance, power devices in the resonant converter are capable of zero-voltage or zero-current switching. As a result, the switching loss of the power devices is reduced, and the overall conversion efficiency enhanced. Recently, with the advancement of manufacturing techniques for resonant controller chips and power devices, and coupled with their declining prices, resonant converters have become increasingly popular in the industry and are extensively used in various types of electronic equipment and instruments. In particular, series resonant converters are especially favored for their high input voltage, high efficiency, and a wide output voltage range.
Referring to FIG. 2, a series resonant converter commonly used in the industry typically includes an input voltage filter capacitor Cin; a resonant controller chip IC; a first power switch Q1; a second power switch Q2; a resonant inductor Lr; a resonant capacitor Cr; a transformer T1; two secondary rectifier diodes D1, D2; and an output voltage filter capacitor Cout. The input voltage filter capacitor Cin is connected across the positive and negative ends of an input voltage Vin. The first power switch Q1 and the second power switch Q2 are series-connected to each other and parallel-connected to the input voltage filter capacitor Cin. The gate of the first power switch Q1 and the gate of the second power switch Q2 are connected to the corresponding control pins of the resonant controller chip IC, respectively. For instance, if the resonant controller chip IC is the high-voltage resonant controller ST L6599A produced by the world-famous chip manufacturer STMicroelectronics, the gates of the first and second power switches Q1, Q2 are connected to the control pins HVG and LVG of the resonant controller chip IC, respectively. In addition, the drain and the source of the first power switch Q1 are connected to the positive electrode of the input voltage filter capacitor Cin and the drain of the second power switch Q2, respectively, while the source of the second power switch Q2 is connected to the negative electrode of the input voltage filter capacitor Cin. Thus, the input voltage filter capacitor Cin is capable of providing a stable input voltage to the transformer T1. The transformer T1 is configured mainly for isolation and includes a primary winding Np and two secondary windings NS1, NS2. The primary winding Np has one end connected to the positive electrode of the resonant capacitor Cr and the other end connected to the corresponding sensing pin OUT of the resonant controller chip IC via the resonant inductor Lr and the line between the two power switches Q1, Q2. The negative electrode of the resonant capacitor Cr is connected to the source of the second power switch Q2. The secondary winding NS1 has one end connected to the positive electrode of the output voltage filter capacitor Cout and the other end connected to the negative end of the secondary rectifier diode D1. Similarly, the secondary winding NS2 has one end connected to the positive electrode of the output voltage filter capacitor Cout and the other end connected to the negative end of the secondary rectifier diode D2. The positive ends of the secondary rectifier diodes D1, D2 are connected to the negative electrode of the output voltage filter capacitor Cout, respectively. Thus, the output voltage filter capacitor Cout is capable of providing a stable DC output voltage Vout to the load connected across the output ends. The working principle of such a conventional series resonant converter is briefly stated as follows. By virtue of the impedance properties of the resonant inductor Lr and the resonant capacitor Cr series-connected on the primary side, the resonant controller chip IC controls the switching frequencies of the two power switches Q1, Q2 and thereby enables the series resonant converter to provide a stable output voltage according to the load connected across the output ends.
Traditionally, referring again to FIG. 2, the conventional series resonant converter is so designed that it can operate in a particular peak-load state, with a view to being applicable to a variety of electronic instruments that tend to generate temporary peak loads. For instance, a series resonant converter with a rated load of 100 W is often designed to be operable under a peak load of 200 W. However, such an expedient design is intended only to enable a series resonant converter, under the conditions of low design and production costs, to provide the high power required for an electronic instrument to activate certain special functions that cause temporary peak loads, such as the temporary peak load occurring when a facsimile machine starts its heat sensor. To prevent the series resonant converter from overheating or damage which may otherwise occur due to continuous operation in the peak-load state, a conventional solution is to add a detection circuit 30 to the primary side of the series resonant converter, as shown in FIG. 3, wherein the detection circuit 30 is configured for sampling a voltage ripple of the resonant capacitor Cr. This is because the higher the load at the output ends on the secondary side of the series resonant converter is, the larger the voltage ripple sampled by the detection circuit 30 will be. After the two rectifier diodes in the detection circuit 30 perform full-wave rectification on the sampled voltage ripple, a DC detection level in proportion to the magnitude of the load on the secondary side is obtained. The detection circuit 30 then transmits the DC detection level to an over-current detection pin ISEN of the resonant controller chip IC. Referring to FIG. 4, if the resonant controller chip IC is the high-voltage resonant controller ST L6599A manufactured by STMicroelectronics, a comparator OCP in the resonant controller chip IC compares the DC detection level with a reference level (e.g., 0.8V). Therefore, if the over-current detection pin ISEN of the resonant controller chip IC detects that the DC detection level exceeds the 0.8V reference level of the comparator, meaning that the series resonant converter is operating in the peak-load state, the comparator OCP will send a control signal to a control circuit Control Logic in the resonant controller chip IC. Consequently, the control circuit Control Logic starts to charge a first external capacitor C41 through a delay pin DELAY of the resonant controller chip IC. In other words, delay timing of the peak load begins. Meanwhile, the control circuit Control Logic turns on a switch Q41 in the resonant controller chip IC so as to discharge a second external capacitor C42 through a discharge pin CSS of the resonant controller chip IC, thereby increasing the operating frequency. As a result, the output power of the series resonant converter is lowered instantly.
However, although the high-voltage resonant controller ST L6599A of STMicroelectronics, when serving as the resonant controller chip IC, has an overload protection mechanism, activation of this overload protection mechanism will cause the operating frequencies of the two power switches Q1, Q2 to rise at the same time, thus limiting the output power of the series resonant converter immediately. Should that happen, the series resonant converter will be unable to maintain the high power required by the peak load at the output ends and therefore cannot provide the high power needed for an electronic instrument to temporarily activate certain special functions. In other words, the conventional series resonant converter cannot realize the mechanism that it should be turned off only after continuous operation in the peak-load state for a predetermined delay time. In addition to the high-voltage resonant controller ST L6599A made by STMicroelectronics, resonant controller chips with delay and power-limiting mechanisms include the high-voltage resonant controller TEA 1713 manufactured by the well-known chip manufacturer NXP Semiconductors. The high-voltage resonant controller TEA1713 has the same functional design as ST L6599A in that, when the resonant controller detects a peak load at the output ends of the series resonant converter, an internal timer starts to time, and the operating frequency is immediately increased such that the output power of the series resonant converter is instantly suppressed to a lower level.
It is apparent from the foregoing that the conventional series resonant converters simply cannot rely on commercially available resonant controller chips to realize the mechanism that the series resonant converters should be turned off only after it is operated continuously in the peak-load state for a predetermined delay time. As a result, many electronic instruments using the conventional series resonant converters tend to have problem in temporarily activating certain special functions that give rise to peak loads. Therefore, the issue to be addressed by the present invention is to make a series resonant converter by incorporating commercially available resonant controller chips into a simple circuit design that not only allows the series resonant converter to operate continuously in a peak-load state as needed, but also turns off the converter immediately after an electronic instrument using the converter successfully and temporarily activates certain special peak-load functions (i.e., after the series resonant converter is operated in the peak-load state for a predetermined delay time), thereby protecting the series resonant converter from overheating or damage which may otherwise result from sustained overload.